1. Field of the Invention
This invention relates to an exhaust gas recirculation apparatus, and more particularly, to an exhaust gas recirculation apparatus suitable for an engine with a turbocharger.
2. Description of the Related Art
It is necessary to reduce the amount of carbon monoxide CO, hydrocarbon HC, and nitrogen oxide NOx contained in exhaust gases from vehicles such as an automobile. It is known that among these poisonous gases, nitrogen oxide NOx can be reduced by mixing inactive gases contained in exhaust gases such as H.sub.2 O, N.sub.2, and CO.sub.2 into the combustion mixture and lowering the maximum combustion temperature by virtue of the heat capacity of the inactive gases. With this fact in mind, in order to reduce nitrogen oxide, an exhaust gas recirculation (hereinunder referred to as "EGR") apparatus which recirculates exhaust gas to an intake system is provided in various kinds of vehicles.
Of these apparatus, in an engine with a turbocharger there is an apparatus in which a bypass pipeline for connecting an exhaust line on the upstream side of an exhaust turbine and an intake line on the downstream side of a compressor is provided as a passage for recirculating exhaust gas, namely an EGR passage, and midway in the EGR passage is provided a valve body for opening and closing the EGR passage in correspondence with the urging force of a diaphragm, namely an EGR valve. In this apparatus, when the engine is in a running state in which exhaust gas recirculation control is necessary, namely when the running state of the engine is in an EGR control range, a vacuum pressure from a vacuum source which sucks the diaphragm of the EGR valve is introduced to a diaphragm chamber, and the EGR valve is opened to open the EGR passage, whereby the exhaust gas is recirculated to the intake system. Except when the engine is in the EGR control range, the EGR valve is closed by the urging force of the diaphragm and the EGR passage is closed.
When an engine is in a high-speed and high-load state, the exhaust gas pressure increases with the increase of the load. On the other hand, in this kind of engine with a turbocharger, in order to keep boost pressure constant, when the boost pressure is beyond a predetermined pressure, exhaust gas is released to a passage detouring the turbine of the turbocharger. As a result, even if exhaust gas pressure becomes high, the boost pressure, namely intake pressure does not exceed a predetermined value. Therefore, the difference of the pressure between the intake side of the EGR passage which connects the intake system passage with the exhaust system passage and the exhaust side thereof becomes large. Furthermore, a vacuum adjustment valve is connected to the EGR valve which is provided in the EGR passage for the purpose of controlling the amount of EGR. This vacuum adjustment valve supplies a vacuum from a vacuum source after controlling it in accordance with the electric current supplied from a controller. However, due to its inherent structure, the vacuum leaks even when the current value is zero, and this leaked vacuum is transferred to the EGR valve as a residual pressure.
Therefore, the conventional EGR apparatus has the deficiencies or problems that the pressure difference between the intake system passage side and the exhaust system passage side is large even if the EGR passage is closed by the EGR valve in the high-speed and high-load range, which is out of the EGR controlling range, and that the EGR valve is inadvertently slightly opened due to the residual pressure of the vacuum adjustment valve and exhaust gas flows to the intake system passage side, whereby the power of the engine is lowered or black smoke is generated.